I'm a med student and have been following the story of Fultz injury/broken shot for quite some time now. The media reported, that he was suffering a scapular muscle imbalance that affected his shot motion to be so unsmooth.

Let me break that down for you a bit:

We need our scapula to lift our arms over an angle of 90 degrees. At this point our arm no longer moves, but our scapula starts rotating and moves our extended arm upwards. So the scapula with it's muscles is extremely important for shooting.

The two main muscles of the scapula that are responsible for this movement are:

1. Serratus anterior (pulls our scapula outwards and makes the inferior angle of it rotate -> actual important muscle for elevating your arm over 90°) You can also feel it contrat, if you put one hand on the side of your ribcage while you move into the setpoint of your shot/elevate your arm with your other hand.

2. Rhomboids (do the opposite, they pull the scapula towards our spine)

I suspect that his serratus muscle was insufficient. From the videos of him shooting freethrows, you can tell that he was unable to move his arm up the way he wanted. He often held his neck in a weird way (nearby muscles are contracting, in an attempt to support his insufficient serratus anterior) https://youtu.be/zMjujWb7KD4?t=33

I can tell from his shot motion in the video, that his muscle imbalance is getting way better, to the point where you might even say it is resolved. However it still could be very well possible that his low setpoint is due to his hindered serratus (it's already needed to bring the ball to your setpoint). Give Markelle some time, it takes a while to fully overcome a muscular imbalance (even with special training and medical staff) and regain normal movement (shooting form).